The invention relates to an improved method for autofocusing a microscope with continuously or incrementally variable objective magnification and microscope systems that utilize the improved method.
In conducting inspections using a microscope, it is often necessary to be able to quickly change magnifications, whereby use is made of objectives having magnifying powers which vary over a wide range, for example, from 5x to 150x and above. Microscopes are widely used for supervising production processes, i.e., for routine examinations, for example, in the production of wafers, and visual focusing is extremely tiresome for the microscope operator. Increasing efforts have been made to employ autofocusing systems which make it possible to quickly focus automatically on the object and, in some instances, also more exactly than is visually possible. However, it has been found that conventional autofocus systems--in particular, when a radiation is used which ensures that the semi-conductor wafer under examination will not be damaged or changed--are not satisfactory in performance when the objective magnification varies over a wide range as noted above. For one, this is due to the fact that the structures inherent in the object or generated on the object get blurred with respect to edge contrast as the objective magnification increases. Accurate focusing deteriorates if this contrast, or a function derived therefrom, such as the value of high spatial frequencies, is used as the focusing criterion. The applicant has found that a further problem resides in the fact that the image-side depth of focus increases as the magnification increases, i.e., it increases by the square of the objective magnification. This fact, no doubt, contributes to a considerable extent to the sometimes inexplicable failure of conventional autofocus systems at different magnifications. For this reason, the applicant has provided a method for autofocusing a microscope and a microscope with autofocus system, which are described in German patent application No. P 37 07 487.4 and U.S. application Ser. No. 50,267, filed May 14, 1987. Said method and microscope ensure reliable and optimal automatic focus adjustments even when the objective magnifications are very different from each other. It has been found that this way of autofocusing is fully satisfactory so long as sufficient details are visible on the object. In particular, in incident light microscopy it is often necessary to focus on object areas having very little detail (e.g., wafers in a first coating stage in the electronics industry) or, in extreme cases, even to focus on pure front-surface mirrors.
Even if sufficient details are visible at a high magnification, rapid movement of the object stage causes the image to shift too quickly through the optimum focus to be perceived by the electronics as sharp or unsharp.